In article <3lgf2r$dui at mserv1.dl.ac.uk>, <W.G.VAN.DOORN at ATO.AGRO.NL> wrote:
> The discussion about the definition of aging between Patrick O'Neil, Oliver
> Bogler and Andy Groves prompted me to give a short thought. I wonder
> whether immortal cells have been observed. Of course, there are cell lines
> that keep on multiplying, hence can be considered, for all practical
> purposes, to be immortal. But have the individual cells in these lines
> been followed? Did they go on dividing since the beginning or did they
> die after a number of replications? In bacterial colonies growing on agar
> there are a high number of dead cells within a few days of growth. The
> colony keeps growing, but the growth is due to young cells. Immortal cell
> lines, therefore, should be compared to species: they will go on multiplying
> until extinction.
>
I'm not sure I understand the point you are making. If you found an
immortalised cell line from a single cell (which is often the case), then
the cell line that arises must presumably be formed from the progeny of
that founder, which has therefore, continued to divide.
In the animal, there are a number of cell populations that continue to
divide throughout the life of the animal - for example, stem cells in the
skin, gut or bone marrow. For example, the human gut sheds about
10,000,000,000 cells every day, which are thought to be replaced by the
division of asymmetrically dividing stem cells in the base of each
intestinal crypt. It has been estimated (but not proven) that there is
probably only one such stem cell in each crypt. The cell divides,
producing a copy of itself and a more differentiated progeny cell, which
will help replace the cells in the intestinal wall. Similarly for bone
marrow and skin. So these are cells which divide throughout the life of an
animal. Of course, your original point remains unanswered, as no one has
followed a single stem cell. It is far easier to infer the existence of
stem cells from kinetic studies than to actually isolate them, although
this has been possible for haematopoietic stem cells in bone marrow.
Andy
--
Andy Groves
Division of Biology, 216-76
California Institute of Technology